Analyzing chemical composition of samples is important in many contexts, including identifying and segregating metal types in metal recycling facilities, quality control testing in factories and forensic work. Several analytical methods are available. One common analysis method employs x-ray fluorescence (XRF). When exposed to high energy primary x-rays from a source, each atomic element present in a sample produces a unique set of characteristic fluorescence x-rays that are essentially a fingerprint for the specific element. An x-ray fluorescence analyzer determines the chemistry of a sample by illuminating a spot on the sample with x-rays and measuring the spectrum of characteristic x-rays emitted by the different elements in the sample. The primary source of x-rays may be an x-ray tube or a radioactive material, such as a radioisotope.
The term x-rays, as used herein, includes photons of energy between about 1 keV and about 150 keV and will, therefore, include: the characteristic x-rays emitted by an excited atom when it deexcites; bremsstrahlung x-rays emitted when an electron is scattered by an atom; elastic and inelastically scattered photons generally referred to as Rayleigh and Compton scattered radiation, respectively; and gamma rays in this energy range emitted when an excited nucleus deexcites.
When exposed to high energy primary x-rays from a source, each atomic element present in a sample produces a unique set of characteristic fluorescence x-rays that are essentially a fingerprint for the specific element. An x-ray fluorescence analyzer determines the chemistry of a sample by illuminating a spot on the sample with x-rays and measuring the spectrum of characteristic x-rays emitted by the various elements in the sample. The primary source of x-rays may be an x-ray tube or a radioactive material, such as a radioisotope.
At the atomic level, a characteristic fluorescent x-ray is created when a photon of sufficient energy strikes an atom in the sample, dislodging an electron from one of the atom's inner orbital shells. The atom then nearly instantaneously regains stability, filling the vacancy left in the inner orbital shell with an electron from one of the atom's higher energy (outer) orbital shells. Excess energy may be released in the form of a fluorescent x-ray, of an energy characterizing the difference between two quantum states of the atom.
By inducing and measuring a wide range of different characteristic fluorescent x-rays emitted by the different elements in the sample, XRF analyzers are able to determine the elements present in the sample, as well as to calculate their relative concentrations based on the number of fluorescent x-rays occurring at specific energies. When samples with known ranges of chemical composition, such as common grades of metal alloys, are tested, an XRF analyzer can also identify the sample by name, by referencing a programmed table or library of known materials.
It is important to note that, except in special circumstances, low concentrations of light elements (low proton or Z number elements) cannot typically be measured directly with portable XRF analyzers, because fluorescent x-rays with energies below about 2.5 kiloelectron volts (keV) are absorbed within short path lengths of air. For this reason, light element XRF analysis requires either a helium gas purge or the evacuation of the volumes through which the relevant x-rays pass.
The size of the x-ray spot on the sample determines how much of a sample is analyzed. In some cases, a small x-ray spot would be desirable. For example, a user who analyzes small components and solder joints on printed circuit boards (such as in recycling facilities) would benefit from using a hand-held XRF analyzer that produces a very small spot, so a component or solder joint of interest could be analyzed without the fluorescing x-rays illuminating surrounding components and, therefore, confusing the chemical composition of the item under test with the chemical compositions of surrounding materials. In another example, analyzing a series of small, spaced-apart portions of a sample would enable a user to gauge the homogeneity of the sample.